The contemporary wind turbine principally employs a doubly-fed induction generator (DFIG) as the major wind generator. When the power grid is abnormal and the voltage of point of common connection (PCC) is sagged, the doubly-fed induction generator has to keep connected to the grid until the grid returns to the normal state, thereby riding through this low-voltage period (zone). Therefore, the doubly-fed induction generator generally is required to have the function of low-voltage ride-through (LVRT). Nowadays, the most common LVRT technique applied to the doubly-fed induction generator is active crowbar circuit. The active crowbar circuit is used to protect the rotor-side circuitry of the doubly-fed induction generator by shorting it and absorbing the inrush current flowing in the rotor-side circuitry of the DFIG when the voltage of the grid is sagged and a transient over-voltage condition and over-current condition are occurred to the rotor-side circuitry of the DFIG accordingly.
The conventional active crowbar circuit is connected in parallel with the rotor of the doubly-fed induction generator, and includes a rectifier bridge and an IGBT (insulated-gate bipolar transistor) switch as well as a resistor for absorbing the inrush current. Each rectifier arm of the rectifier bridge is consisted of two serially-connected diodes, and the IGBT switch and the absorption resistor are placed on the DC side of the active crowbar circuit. In addition, in order to ensure that the active crowbar circuit can turn on and off normally, the active crowbar circuit has to carry out a self-test procedure after the doubly-fed induction generator is powered on for detecting if the active crowbar circuit can operate normally. The conventional self-test method for active crowbar circuit includes the following types. The first type of the self-test method is to detect if the voltage of the rectifier bridge of the active crowbar circuit is zero. However, the disadvantage of such type of self-test method is that the voltage across the bridge arm is uncertain due to the parasite capacitors when the IGBT switch of the active crowbar circuit is turned off. Therefore, the threshold voltage for testing if the IGBT switch is turned on is difficult to design, and a voltage sensor is needed for voltage detection. The second type of self-test method is to detect the current of the active crowbar circuit to determine if the IGBT switch of the active crowbar circuit is turned on. However, such type of self-test method also has some disadvantages. This is because the resistance of the active crowbar circuit is generally very small, e.g. smaller than 1Ω. If the rotor-side converter and the IGBT switch of the active crowbar circuit are turned on simultaneously by the PWM signal, it will cause the over-current problem to the rotor-side converter.
Hence, the applicants endeavor to develop a doubly-fed induction generator system and a self-test method for the active crowbar circuit thereof. With the invention, the active crowbar circuit of the doubly-fed induction generator system can be self-tested without causing the over-current problem to the rotor-side converter of the doubly-fed induction generator.